Tuesday 15 March 2005

Tree Roots Part 1

Get Back to your Roots Laddie!

The truth of this statement by a Scottish Kilt salesman, leads us into the largely unseen world of roots.

Roots are the unsung heroes of the plant world. Without the obvious utility of branches or the chromatic charisma of the leaves roots work tirelessly year round.

Roots provide 3 basic functions to a tree:

1) Physical anchoring the above ground portions of the tree

2) Absorbing and transporting water and mineral nutrients

3) Storing of starches, sugars and other food reserves

In addition to these 3 basic properties, roots synthesize important bio-chemicals that regulate how the above ground parts of the tree function.

There are many different types of roots. Trees typically have one or two basic types.

Tap roots are deep probing roots that serve to anchor trees and gain access to deep water resources. Tap roots may extend several meters directly below the trunk of the tree. The burr oak is the classic example of a tap root species. This deep root will be accompanied by many shallower fibrous roots and enable burr oaks to resist drought. Tap roots are the exception in the world of trees; fibrous roots are much more common. Tap roots normally die off and are replaced with lateral roots as trees mature.

Trees like the white spruce and Manitoba maple are good examples of fibrous rooted trees. Large numbers of shallow branched roots extend laterally from the base of fibrous rooted trees. Most often 90% of roots are in the top 150cm, or 6 inches of the soil. This area is called the rhizosphere, in English “the sphere of roots” or “root world”. Roots rely on three essential things in proper proportion in order to grow and thrive. Oxygen, found in the soil atmosphere, mineral nutrition in the form of soil particles, and water are the three basic requirements for healthy roots. Organic material constitutes a forth and smaller component of the soil. The rhizosphere has these components in a balanced proportion.

Pores in the soil are created between soil particles and by living soil organisms. These organisms digest soil particles and each other creating organic matter essential to tree growth. The proportions move in and out of balance as rain water fills pores in the soil and displaces air. Soil compaction can also disrupt the amount of air and water in the rhizosphere. Large tree roots are mainly structural and do not absorb substantial amounts of water or nutrients. The smallest roots are the water adsorbing roots, doing the vast majority of nutrient absorption. These small roots grow and fall off in response to seasonal changes in the soil climate. Winter cold or long periods of saturated soil will result in the loss of many of these small fine roots. Replacing them in the spring or when soil conditions are favorable to root growth is an energy intensive business. All this activity goes on largely unseen by the average person. Things we do above ground will often have negative effects on the delicate balance of the rhizosphere. We will dig deeper into root world in our next column, as we discuss root damage during construction.